Infectious diseases are a major cause of global morbidity and mortality in neonates and infants. The heightened susceptibility to infectious diseases over the first year of life, and difficulties inducing rapid and protective vaccine responses, are largely due to an immature immune system. Significant gaps in knowledge exist in understanding how to enhance protective immune responses soon after birth and throughout infancy. There is growing recognition that an individual's immune response to one pathogen can be shaped by the prior exposure history to unrelated pathogens, and this has been termed heterologous immunity. Heterologous immune responses are likely to play a significant role in shaping and maturing the malleable immune system over neonatal life and early infancy. Bacillus Calmette-Guerin (BCG) is a live attenuated mycobacterial vaccine typically given within the first two weeks of life, and it is the most widely used vaccine in the world. In addition to providing clinical protection against disseminated forms of tuberculosis, BCG vaccination has also been reported to reduce overall infant mortality and suggested to have beneficial heterologous immune effects. The specific aim of this exploratory proposal is to delineate if neonatal BCG vaccination alters the specific immune responses to subsequent unrelated infant vaccines. The proposed study will begin to delineate heterologous immune effects of neonatal BCG vaccination. A nested case-control study will be conducted comparing T-cell and antibody responses to the first (priming) doses of tetanus toxoid, live attenuated oral polio virus, and hepatitis B vaccination in infants with prior neonatal BCG vaccination and those without neonatal BCG vaccination. The study will utilize existing peripheral blood mononuclear cells (PBMC) and plasma samples that have been collected as part of an ongoing prospective clinical study of dengue virus infections during infancy. The existing blood samples are from infants without dengue virus infections and would not otherwise be used in the dengue study. Vaccine antigen-specific T-cell functions and phenotypes will be assessed by state-of-the-art flow cytometry assays in the PBMC samples. IgG antibody responses will be measured by ELISA in the plasma samples. The findings in this exploratory study will support the development of a more definitive study to delineate the effects and underlying mechanisms of heterologous immunity in neonates and infants. This proposal is a first step towards a better understanding of human immunology in the newborn and developing new approaches to neonatal and infant vaccination. Expanding and improving the ability to induce protective immunity against infectious diseases in the first few months of life will have a significant impact on global health.